Trackball ball and sewage treatment plant: what they have in common

1. Step 1. Trackball ball — is part of — Trackball. It is spun with the fingers.
2. Step 2. Trackball — has part — Ball bearing. The ball rotates on supports.
3. Step 3. Ball bearing — is part of — Bogie. Axle boxes with roller bearings.
4. Step 4. Bogie — is part of — Refrigerator car. Bogies.
5. Step 5. Refrigerator car — has part — Compressor. A refrigeration unit.
6. Step 6. Compressor — is part of — Sewage treatment plant. Blowers supply oxygen to the microbes.

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